Four-wheel steering systems are receiving increasing attention from car manufacturers and from consumers. Four-wheel steering substantially increases the turning radius of a vehicle and increases high-speed handling. When a vehicle negotiates a turn, inertia urges the vehicle to continue in a straight line. With conventional front-wheel steering, the rear tires tend to slip away from the direction of turn of the front wheels. Slip angle of a tire is defined as the angle between the line of actual travel and the direction in which a wheel is pointed. Slip angle is obviously different between the rear wheels and the front wheels of front-wheel steering vehicles, since the rear wheels are locked in a position parallel to the longitudinal axis of the vehicle.
Slip angle increases if more cornering force is demanded, but slip angle is normally limited to a maximum of between ten and fourteen degrees, depending upon the tire design and the road surface. The force that keeps a vehicle on a road is the friction between the tire and the road, and a slip angle beyond fourteen degrees will overcome this friction to send a vehicle into a slide.
Even a small change in the angle of the rear wheels relative to the longitudinal axis of the vehicle will have a large effect upon the cornering characteristics of a car since slip angle is reduced. Vehicles having four-wheel steering are known. U.S. Pat. Nos. 2,910,131 to Krotz, 3,255,840 to Tangen, 4,289,214 to Spence, 4,337,840 to Bufler and 4,592,439 to Collard et al. disclose vehicles in which all four wheels are turned. But while four-wheel steering produces a quicker response, it is incapable of changing physical laws and, therefore, the limits of friction and the cornering force of tires remain unaffected.
High cornering forces create car body roll, thereby redistributing the weight toward the wheels that are radially outward of a steering center. Thus, side stress problems are a concern. Additionally, a wheel which is deflected by body roll from a normal axis of rotation will influence the vehicle's direction. When both front wheels or both rear wheels are deflected by roll, the roll-steer effect can be pronounced. The result is a vehicle which is less comfortable for the occupants of the vehicle.
A vehicle that changes direction well is considered to be a good handling vehicle, whereas a vehicle that does not change direction under certain conditions is termed as a stable vehicle. Both are desirable characteristics. Four-wheel steering improves the handling of a vehicle but it does not follow that stability is improved. Moreover, four-wheel steering does not, by itself, compensate for the variations in wheel rotational speed during turns. Such variations produce asynchronous gyroscopic forces.
An object of the present invention is to provide a steering and suspension system which improves the comfort and stability of the vehicle in addition to improving vehicle handling.